Field of the Invention
The present invention relates generally to a wafer retaining mechanism
for semiconductor processing equipment, and more particularly, to wafer holder
apparatus used in batch type ion implanters.
Background of the Invention
In batch type ion implanters a number of wafers are mounted on a
cooled platen forming part of a spinning implant disc and exposed to the ion beam
for doping semi-conductor wafers. This arrangement allows increased throughput
by using a high current ion beam while maintaining the temperature of the wafers
within the acceptable range.
Many efforts have been made to improve ion implantation technique
by elimination of contamination and cross-contamination.
One source of contamination is sputtering of accumulated ion species
from wafer holders exposed to the ion beam onto the wafer. These species function
as cross-contaminants of previous implantations.
Another source of contamination is particles falling on the wafer
during wafer processing. Most particles are produced by improper clamping of the
wafer. The particles disposed on the wafer surface before the implantation process
screen the implantation. The particles which fall on the wafer after implantation
cause contamination of the following processed wafer.
Early devices for retaining wafers during ion implantation utilized
either peripheral clamping mechanisms, for example, devices with a clamp ring engaging
the front surface of the wafer or centrifugal clamping apparatus to hold the wafer
on the cooled platen.
Along with the desire to prevent particulate formation and cross
contamination in both types of clamping, the wafers must be protected from physical
damage during the process. To avoid breakage of the wafers, they should be accurately
positioned at the disc before clamping them thereon.
Peripheral clamping devices such as a clamp ring obstructs part of
the front surface of the wafer and are susceptible to particle generation effects.
Because of build-up of sputtered material on the clamp ring, the ring can be a
source of particulate contamination. During the implantation process, the clamping
ring is exposed to the ion beam and becomes a source of sputtered contamination
from the clamping ring to the surface of the wafer. Moreover, the shadowed portion
of the wafer is not exposed to the implantation process.
Centrifugal clamping techniques eliminate the need for clamping members
that contact the wafer surface, and reduce the amount of wafer holder exposed to
the ion beam above the wafer's plane. A centrifugal clamping arrangement provides
the whole surface of the wafer for ion treatment, but specific mechanically complicated
disc structure is required for producing sufficient centrifugal force for clamping.
One attempt to provide better utilization of the wafer surface and
reduce both particle generation and sputter contamination effects was disclosed
in U.S. Patent No. 4,817,556 "Apparatus for Retaining Wafers" assigned to the assignee
of the present invention. This mechanism is located primarily on the backside of
the platen and uses edge handling. A flexible collet with a number of fingers around
the periphery of the wafer retains the wafer by contacting its edge protruding
above the surface of the platen. The platen has a plurality of grooves around its
periphery, and each groove accommodates a finger. The outside portion of the collet
is molded into and supported by an elastomer ring so as to allow the collet to
flex, moving the fingers outward so that the wafer can be loaded onto the platen.
The multiple grooves in the platen expose the flexible collet and its elastomer
supporting ring to the ion beam. When the collet is actuated the material deposited
by the ion beam, as well as the wear particles generated in the mechanism, are
ejected and can deposit on the front surface of the wafer.
A multi-axis robotic system which is used in U.S. Patent No. 4,817,556
for loading wafers onto the platens of the implant disc creates a certain amount
of positioning error, particularly when operating inside a large vacuum chamber
where perfect alignment is difficult to achieve. The rather limited movement of
the fingers provided by the flexing of the collet makes this design sensitive to
these positioning errors. Any misclamped wafer could be broken during the implant
process causing loss of valuable material and expensive downtime to clean the implanter.
Therefore, reliable operation of the wafer retaining mechanism could be provided
by including wafer misclamp detection on systems, so that misclamped wafers could
An attempt to solve the positioning problem is described in U.S.
Patent No. 4,744,713 "Misalignment Sensor for a Wafer Feeder Assembly" detects
wafer position to prevent misclamping of the wafer by sensing a collimated light
beam reflected off the surface of the wafer. To implement the design a light source
and a light sensor have to be placed on a supporting structure inside the vacuum
chamber of the ion implanter far enough from the surface of the wafer to allow
the disc transition from the wafer loading position to the implant position. To
ensure reliable detection of misclamped wafers, a precise alignment of the implant
disc and the components of the sensing system has to be maintained. Though the
device effectively provides security against misalignment in the mounting of the
wafer and avoids possible physical damage to the wafer, the clamping of the wafer
is maintained with a peripheral clamping mechanism. This wafer clamping system
includes a retractable spring loaded clamp with two arms which engage the wafer
when loaded, wherein, in operation part of the wafer is overshadowed by the arms.
The arms being exposed to the ion beam accumulate species which become cross-contaminants
in a following implantation process.
Our earlier patent specification US-A-5040484 discloses apparatus
for retaining a wafer on a disc, said wafer having first and second opposed edges
and flat surfaces in front and back, comprising a platen for supporting the wafer,
clamp means mounted on the platen for relative reciprocating motion, and pusher
means mounted for reciprocal motion, moving in one direction to cause the clamp
means to release the wafer and in the other direction out of direct connection
therewith so that the clamp means holds the wafer on the platen, together with
lifting means coupled to said pusher means for lifting said wafer from said platen
after the clamp means has released the wafer. The present invention improves on
this arrangement by providing the platen with a fence for contacting the first
edge of the wafer and by providing lever means pivotally attached to the clamp
means, spring means attached to said lever means for urging said clamp means to
contact the second edge of said wafer to press said first edge against said fence
of said platen, said pusher means moving in said one direction to move said clamp
means away from said fence to release the wafer and in the other direction out
of engagement with the lever means so that the clamp means holds the wafer against
said fence of said platen.
Other aspects of the present invention will become apparent from
the following detailed description, taken in conjunction with the accompanying
drawings illustrating by way of example the principles of the invention.
Brief Description of the Drawings
Detailed Description of the Invention
- FIG. 1 is a detailed cross sectional view of an apparatus for releasably holding
wafers on a spinning disc of a batch ion implanter constructed according to the
- FIG. 2 is a cut-away partial perspective view of the apparatus for retaining
wafers according to the present invention.
- FIG. 3 is a schematic block diagram of a wafer detection sub-system for use
with the wafer retaining mechanism of the invention.
FIG. 1 depicts a wafer-holder apparatus which includes a wafer-receiving
platen 1. Platen 1 is installed in spinning disc 8 of a batch processing ion implantation
system and having a plurality of through holes (not shown). Platen 1 has a flat
front surface exposed to the ion implant beam and a rear surface mounted on the
spinning disc 8. Fence 1A (shown in FIG. 2) is configured as a raised circular
portion on the periphery of the front surface of platen 1 to match the circumference
of the wafer 2 so as to engage a side surface of the wafer 2.
The apparatus further comprises pull rod 6 which is installed in
guide bushing 14. The first end of pull rod 6 is bent outwardly toward the front
surface of platen 1. Roller 7 is mounted on the first end of pull rod 6 so that
roller 7 and the first end of pull rod 6 extend above the front surface of the
platen 1 so as to engage the edge of the wafer 2. The second end of pull rod 6
is attached to pivoting lever 5 which is supported by housing 3. Lever 5 rotates
around pivot point 15. Spring 4 is positioned within housing 3. Spring 4 provides
the force required to move the wafer 2 towards fence 1A of the platen 1 and retain
the wafer 2 while disc 8 is in a vertical position. Finger 9 extends from the side
of lever 5 below the spinning disc 8 opposite the side to which pull rod 6 is attached.
Counterweight 10 is attached to the underside of pivoting lever 5. The apparatus
further comprises spring loaded pusher mechanism 11 coupled with wafer lift plate
13 mounted on drive shaft 16. Lift pins 12 are mounted on wafer lift plate 13.
In operation, wafers are loaded and unloaded in the implant chamber
when the disk is in a horizontal position. Disc 8 is rotated so that the wafer
platen is in position to receive a wafer which is conveyed to the disc. An example
of an implantation system with a mechanism suitable for loading the wafers onto
a platen and an overview of typical operations is provided in U.S. Patent No. 4,817,556,
assigned to the assignee of the present invention, which is incorporated herein
After loading of wafers is completed, disc 8 is pivoted into the
vertical plane for implant, approximately perpendicular to ion beam, and then disc
8 is spun. Roller 7 presses wafer 2 against fence 1A by the force of spring 4.
Disc has to be brought up to speed. When the spinning speed achieves the value
of approximately 400 rpm and wafer 2 is securely held to the platen by the centrifugal
force, counterweight 10 which is also subject to the centrifugal force will overcome
the bias of the spring 4 and by means of pivoting lever 5 will move pull rod 6
with roller 7 away from wafer 2. When the implant process is completed and the
disc spinning speed is reduced, roller 7 will return to its original position retaining
wafer 2 in place while the disc 8 is being transferred to the horizontal position
for wafer 2 batch reloading.
To open the wafer retaining mechanism, wafer lift plate 13 with lift
pins 12 is raised by a linear actuating mechanism connected to drive shaft 16 (FIG.
1). Pusher 11 first contacts finger 9 and pivots lever 5 extending spring 4 and
moving pull rod 6 with roller 7 away from the edge of wafer 2. Continued upward
motion of the wafer lift plate 13 will cause lift pins 12 to contact and then raise
wafer 2 above the surface of the platen so that wafer 2 can be accessed by a robotic
After a new wafer is delivered to pins 12 by the robotic loading
device, pins 12 lower wafer 2 onto the surface of platen 1. When wafer 2 is placed
on the platen 1 and pins 12 continue their movement downwardly, pusher 11 starts
moving downward with finger 9 following its movement and maintaining contact with
pusher 11 until roller 7 engages and locks wafer 2 against the fence of platen
1. After that, finger 9 stops moving, and the electrical contact between pusher
11 and finger 9 is interrupted. The break in the electrical contact is detected
by wafer handler controller 20, which is a part of conventional detection systems
further including wafer presence sensor 21, servo motor controller 22, and motor
23 shown in FIG. 3. If wafer 2 was delivered to platen 1 with too large an error,
such as being placed over the fence or over the roller, it will not be clamped
and roller 7 will continue its motion to the end of the stroke range with finger
9 maintaining electrical contact with pusher 11. Since the position of the pusher
means is continuously monitored by the wafer handler controller 20 it is possible
to detect whether the electrical contact was interrupted before the end of the
stroke of pusher 11 and so identify a misclamped of wafer 2.
It should also be noted that the geometry of the linkages of the
wafer retaining mechanism is such as to provide close to linear motion of pull
rod 6 decreasing therefor relative motion between roller 7 and the edge of wafer
2. Most of the elements of the wafer clamping apparatus are mounted onto the backside
of the platen to reduce dramatically particulate generation.
Although a specific embodiment of the wafer retaining mechanism has
been described and illustrated, the invention is not to be limited to the specific
forms or arrangements of parts so described and illustrated, and various modifications
and changes can be made without departing from the scope of the invention. Within
the scope of the appended claims, therefore, the invention may be practiced otherwise
than as specifically described and illustrated.